Many modern computer systems utilize magnetic disks as an auxiliary storage medium for data. The disks are divided into adjacent circumferentially extending tracks and each track is, in turn, divided into sectors. Each of the sectors contains a data field storing a preselected amount of data, e.g. 512 bytes. A read/write head is positioned over a preselected track while the disk is rotated to encode and/or decode a sequence of magnetic states into each sector of the track directly beneath the head. In this manner, data may be either written onto the disk or read from the disk as a function of the sequence of magnetic states, as is well known. The width of the head is approximately equal to the width of a track. It is critical that the head be precisely aligned with the centerline of the track being written to or read from to make certain that correct data information is being stored onto or read from the magnetic disk. In the event that the head is not properly aligned with a track centerline, the head will straddle two adjacent tracks and encode or decode invalid data information onto or from the two overlapped tracks.
Typically, a plurality of disks are stacked on a common rotating shaft in an axially spaced relation to one another and a rotable arm mounting at least one head at an outer end is positioned over each of the rotating disks. An electromechanical device, such as a dc limited angle motor and voice coil type actuator, is coupled to the arms. The dc limited angle motor moves the arms to position and hold the heads over the respective disks at a preselected angle such that the heads are each aligned with the centerline of a particular track of a respective disk. Generally, one head at a time is operated to encode or decode its respective disk. In modern computer systems, the density of the data stored on the disks is very high resulting in extremely minimal dimensions for the width of each track. Accordingly, it is extremely important that the control mechanism for the limited angle motor operates to precisely control the angular position of the mounted head to coincide with the centerline of a track.
Precise control of the operation of the limited angle motor is obtained by utilizing a servo mechanism. A servo mechanism generally comprises a control mechanism for a device wherein an error signal derived from a summation of a reference input and an actual output is fed back to the control mechanism to correct the performance of the controlled device. In the context of a disk drive, the actual angular position of a head is summed with the desired angular position for the arm mounting the head and the difference is utilized to control the limited angle motor to move the head to the correct angular position. Thus, the limited angle motor is energized to change the angular orientation of an arm until the actual angular position of the head equals the desired angular position, i.e., the error signal is zero.
In accordance with a known servo mechanism for a computer disk drive, position information relating to the head is embedded or written directly on the tracks of each disk between the data fields of adjacent sectors. The head reads the position information as the disk is rotated beneath the head and transmits the embedded information to a track position detector for processing. Accordingly, the head serves as a position transducer component for the servo mechanism. The embedded position information includes A and B bursts encoded on the track adjacent the beginning of each data field. For each data field, one of the bursts is positioned at and above the track centerline and the other burst is spaced in the circumferential direction from the first burst and positioned at and below the track centerline, or vice versa. Each burst comprises a sequence of pulses and the track position detector integrates the signals of the A and B bursts over time to provide integrated A-B and A+B analog signals. The output of the track position detector is defined as A-B/A+B.
If the head is properly positioned over the centerline of a track, the integration value for each of the A and B bursts should approximately equal one another and A-B/A+B should equal zero. The A-B/A+B ratio is a proportionality number representative of a unit of measure of track position from the centerline.
In the embedded servo technology utilized in disk drive servo controls, the AB bursts are position references which permit the head to relate position error from centerline of the track. The AB bursts are encoded on the disks by a servo writer which utilizes an external transducer such as a laser interferometer to precisely position the servo writer head relative to the centerline. However, due to vibrations, spindle bearing runout and other factors, the servo writer does not write the AB bursts exactly on the centerline.
Accordingly, after the encoding of the AB bursts by the servo writer on a track, the servo writer positions itself over the centerline of the track by the interferometer and measures any misplacements of the AB bursts from the centerline caused by the disturbances during the writing of the AB bursts. The measurement information is recorded in each sector of the track adjacent the actual AB bursts as a Servo Correction Number. The Servo Correction Number is read by the head during normal operation of the disk drive and is provided to the servo control system to compensate for any off center readings of the AB bursts caused by misplacements which occurred during the servo writer operation.
A significant problem associated with the embedded servo technology described above is that the system requires a tight control over the track position detector gain and servo control feedback loop gain for satisfactory performance. This is particularly true when the data is stored on the disks in a high density configuration which requires a very precise, high performance control of head position and also, when the disk drive components are mass-produced with normal manufacturing tolerances.